Recently a customer contacted us about increased vibration on a motor, due to a broken washer. And they wanted to know if there was a specification for washer hardness, and a guide for bolt size to washer size.
There are many standards for bolting (USS, SAE, ASME, etc.), of which, washers are an important part. As an example, a structure like a bridge or skyscraper may have a stringent metallurgy requirement, while one for a small industrial motor may not. In industry, the problem is that most purchasing departments specify “washers” – without regard to strength, hardness, malleability, etc.
I have seen broken washers increase machine vibration, since it means that the bolt is now loose. My opinion, and it is only that, is that broken washers are caused by three things:
• They are cheap
• They are improperly sized or installed, including improper sizing, excessive torque, or installing bolts much smaller than the bolt hole it goes in.
• They are deformed or worn, and should be replaced
Washers should be specified, not just purchased because they are the “cheapest”. Hardened washers are available, and can be beneficial. But increasing the hardness of metal normally increases it brittleness as well. Using two standard flat washers may help. Your maintenance or design engineer should be able to help determine the proper application.
Washers are sized. A 3/4″ bolt should use 3/4″ washers. A 1″ washer fits easily on a 3/4″ bolt, but is too big to do its job, which is to:
• provide a flat surface against which the head of the bolt and the face of the washer contact.
• Increase the load area of the bolt.
• Span the hole diameter.
• Prevent damage to the machine foot by the turning bolt/nut.
Washers should normally be installed in this fashion.
In addition, washers which are visibly damaged, deformed, cupped, or rusted should not be used.